According to the law of conservation of mass, mass can neither be created nor be destroyed. Thus the mass of products has to be equal to the mass of reactants. The number of atoms of each element has to be same on reactant and product side. Thus chemical equations are balanced.
The balanced reaction for combustion is as follows ; 2C₂H₅OH + 6O₂ ---> 4CO₂ + 6H₂O the stoichiometry of C₂H₅OH to O₂ is 2:6 that means 2 mol of C₂H₅OH reacts with 6 mol of O₂. when 1 mol of C₂H₅OH reacts with 6/2 mol of O₂, then 0.3020 mol of C₂H₅OH reacts with - 6/2 x 0.3020 therefore number of O₂ moles reacted = 0.91 mol